Conventionally, as a radio frequency power amplifier for amplifying a modulated signal with varying envelope, Class A or Class AB linear amplifier had been used in order to amplify the signal with varying envelope in a linear manner. Whereas these linear amplifiers are excellent in linearity, power efficiency of the linear amplifiers is low as compared with Class C to E non-linear amplifiers. Consequently, when these radio frequency power amplifiers are applied to a mobile communications device whose power source is a battery, due to a large power consumption of the radio frequency power amplifiers, there accrues a problem of a shorter operating time. In addition, when these radio frequency amplifiers are applied to a base station device of a wireless system in which a plurality of transmitter circuits outputting a large amount of power are mounted, problems of increases in size of the device and in power dissipation will arise.
Consequently, as a transmitter circuit capable of operating with high efficiency, a transmitter circuit in which a polar modulation method is employed has been proposed. FIG. 26 is a block diagram illustrating an exemplary configuration of a conventional transmitter circuit 500 in which the polar modulation method is employed. In FIG. 26, the conventional transmitter circuit 500 includes a polar coordinate signal generation section 501, an angle modulator section 502, a power source terminal 503, a regulator 504, an amplitude modulator 505, and an output terminal 506.
The polar coordinate signal generation section 501 generates an amplitude signal and a phase signal. The amplitude signal is inputted to the regulator 504. A DC voltage is supplied to the regulator 504 from the power source terminal 503. The regulator 504 supplies to the amplitude modulator 505 a voltage in accordance with the inputted amplitude signal. And the phase signal is inputted to the angle modulator section 502. The angle modulator section 502 angle-modulates the inputted phase signal and outputs an angle-modulated signal. The angle-modulated signal outputted from the angle modulator section 502 is inputted to the amplitude modulator section 505. The amplitude modulator section 505 amplitude-modulates the angle modulated signal with the voltage supplied from the regulator 504 to output a modulated signal. This modulated signal is outputted as a transmitted signal from the output terminal 506.
And in the transmitter circuit with the polar modulation method employed, distortion of the transmitted signal may occur due to non-linearity of the regulator 504 and the amplitude modulator section 505. For the transmitter circuit with the polar modulation method employed, a method for compensating the non-linearity of the regulator 504 and the amplitude modulator section 505 by using a table has conventionally been disclosed (for example, refer to U.S. Pat. No. 6,366,177). As a conventional transmitter circuit using such a method, there is, for example, a transmitter circuit 600 shown in FIG. 27. FIG. 27 is a block diagram illustrating an exemplary configuration of the conventional transmitter circuit 600. In FIG. 27, a predistortion section 601, based on a transmitted signal, produces a predistortion table so as to compensate the non-linearity of the regulator 504 and the amplitude modulator section 505. An amplitude control section 602 and a phase control section 603, based on the predistortion table produced by the predistortion section 601, previously distorts an amplitude signal and a phase signal to be inputted to the regulator 504 and the amplitude modulator section 505.
In recent years, however, wider modulation bandwidth of a transmitter circuit has been desired and consequently, it is anticipated that frequency bandwidth of the regulator 504 and the amplitude modulator section 505 would become insufficient. For example, when the frequency bandwidth of the regulator 504 and the amplitude modulator section 505 are insufficient, the conventional transmitter circuit 500 (refer to FIG. 26) has a problem such as distortion of a transmitted signal. As one example of such a problem, a result of simulating a case where in the transmitter circuit 500, the frequency bandwidth of the regulator 504 and the amplitude modulator section 505 are insufficient is shown in FIG. 28. In this case, as a modulated signal, a modulated wave, in a π/4-shift QPSK method, having 512 of a symbol number, 128 of oversampling, α=0.5 of a root-raised filter, and 3.84 MHz of a symbol rate is used. And a transfer characteristic between the regulator 504 and the amplitude modulator section 505 is made same as a characteristic of a low-pass filter having a linear attenuation characteristic whose cutoff frequency is 26.4 MHz.
Under the above-mentioned conditions, a value of an adjacent channel power ratio (ACP) in a case where integral bandwidth is 3.84 MHz and detunings are 5 MHz and 10 MHz is calculated and this result is shown in FIG. 28. In FIG. 28, a horizontal axis (in other words, Delay) is a delay time between an amplitude signal and a phase signal and a unit is a number of samples. For example, a delay in one sample corresponds to 2 ns. And an ACP for detuning of −5 MHz is an ACP at a point where a frequency is 5 MHz lower than a center frequency. Judging from this result, it turns out that even when a delay time, caused by a difference in paths, between the amplitude signal and the phase signal is adjusted, ACPs cannot be sufficiently reduced and an optimum delay time for ACPs depends on detuning, making it difficult to sufficiently reduce all ACPs.
In addition, even when in a transmitter circuit 600 (refer to FIG. 27), non-linearity of the regulator 504 and the amplitude modulator section 505 is compensated, it is difficult to suppress distortion caused by insufficient bandwidth of the regulator 504 and the amplitude modulator section 505.
Therefore, an object of the present invention is to provide a transmitter circuit capable of operating with low distortion and high efficiency in a modulation method in which modulation bandwidth is wide and a communications device using the transmitter circuit.